The present invention relates to the field of mass storage devices. More particularly, this invention relates to bearing performance in a disc drive.
One of the key components of any computer system is a place to store data. Computer systems have many different places where data can be stored. One common place for storing massive amounts of data in a computer system is on a disc drive. The most basic parts of a disc drive are a disc that is rotated, an actuator that moves a transducer to various locations over the disc, and electrical circuitry that is used to write and read data to and from the disc. The disc drive also includes circuitry for encoding data so that it can be successfully retrieved and written to the disc surface. A microprocessor controls most of the operations of the disc drive as well as passing the data back to the requesting computer and taking data from a requesting computer for storing to the disc.
The transducer is typically housed within a small ceramic block. The small ceramic block, also referred to as a slider, passes over the disc in a transducing relationship with the disc. The transducer can be used to read information representing data from the disc or write information representing data to the disc. When the disc is operating the disc is usually spinning at relatively high revolutions per minute (RPM). These days common rotational speeds are up to 10,000 RPM. Higher rotational speeds are contemplated for the future. The small ceramic block, or slider, is usually aerodynamically designed so that it flies over the disc. The best performance of the disc drive results when the ceramic bock is flown as closely to the surface of the disc as possible. Today""s small ceramic block or slider is designed to fly on a very thin layer of gas or air. In operation, the distance, or fly height between the slider and the disc is very small. Currently, fly heights are about 0.5-1.0 micro inches. In some disc drives, the slider passes through a layer of lubricant rather than flying over the surface of the disc.
Information representative of data is stored on the surface of the memory disc. Disc drive systems read and write information stored on tracks on memory discs. Transducers, in the form of read/write heads attached to the sliders, located on both sides of the memory disc, read and write information on the memory discs when the transducers are accurately positioned over one of the designated tracks on the surface of the memory disc. The transducer is also said to be moved to a target track. As the memory disc spins and the read/write head is accurately positioned above a target track, the read/write head can store data onto a track by writing information representative of data onto the memory disc. Similarly, reading data on a memory disc is accomplished by positioning the read/write head above a target track and reading the stored material on the memory disc. To write on or read from different tracks, the read/write head is moved radially across the tracks to a selected target track. The data is divided or grouped together on the tracks. In some disc drives, the tracks are a multiplicity of concentric circular tracks. In other disc drives, a continuous spiral is one track on one side of a disc drive. Servo feedback information is used to accurately locate the transducer. The actuator assembly is moved to the required position and held very accurately during a read or write operation using the servo information.
The memory disc rotates upon a spindle. The spindle assembly contains a spindle motor having bearing assemblies such as hydrodynamic bearings or ball bearings. Bearing assemblies are used in numerous applications to allow two surfaces to be supported, guided, or rotated in relation to each other. In a disc drive, the bearing assembly allows the memory disc to be rotated at high speeds. One type of bearing assembly used in disc drive spindle motors are ball bearings. Ball bearings are bearings in which a ball rotates between an inner race and an outer race. One race is attached to a stator, or stationary, machine element. The other race is attached to a rotor, or rotational, bearing element. In a disc drive, the stator is a shaft around which the spindle assembly rotates and the rotor is the spindle assembly upon which the memory disc rests. The ball member in the ball bearing freely rotates relative to the two surfaces, allowing them to pass each other smoothly. Ball bearings are also used in many devices beyond disc drives such as in diverse machinery and automobiles.
One problem that arises when ball bearings are used in high speed applications, such as disc drives, is that the ball may periodically lose contact with the race or races. This results in errors in motion, unwanted vibration and decreased stiffness, or responsiveness, of the bearing. These problems can then lead to the slider hitting the surface of the memory disc and damaging it, or it may lead to the transducer being unable to quickly locate the correct track to perform a read/write operation. Another problem is that there is a trade-off between stiffness of the bearing and life span of the bearing. If the bearing is subject to a high preload, it does not last as long as if it was subject to a low preload. The designer must compromise between these two factors, and cannot reach optimal performance.
One general solution to the problem is to apply a preload force on the bearing to stiffen its performance. Preload is the application of an axial load to a bearing in order to eliminate free radial and axial movement. This solution may temporarily increase stiffness, but bearing performance and the level of preload needed to stiffen the bearing will change over time because of the bearing wearing down. Also, only changing preload on the bearing is ineffective to dampen gyroscopic vibrations of the bearing assembly. Another problem is that the trade-off between stiffness and life span cannot be controlled.
Hydrodynamic bearings are also used in disc drives. Hydrodynamic bearings are bearings in which a fluid gap, such as a lubricant or air, is created between the stator or stationary bearing surface and the rotor or dynamic bearing surface which faces the stator surface. The gap is created when the rotor surface is moving over the stator surface at a high enough speed. The size of the gap between the surfaces affects two variables. If the gap decreases, the stiffness of the bearing is increased, but the power dissipation increases also. However, if the gap increases, the power dissipation decreases, but the stiffness decreases also. The interplay and trade-off of these two variables can be a problem because the performance of the bearing depends on these variables being optimally set at the correct time. For example, the designer may want low power dissipation during normal operating conditions of the bearing, and high stiffness during start up of the bearing. In such a case, the designer is forced to make a compromise decision between stiffness and power dissipation and is left with a situation of less than optimal bearing performance.
Thus, what is needed is a bearing assembly in which the preload can be actively varied to optimize the stiffness/life span tradeoff, in which the preload can be varied to dampen gyroscopic vibrations, and in which the hydrodynamic gap can be varied to optimize the power dissipation/stiffness trade-off.
The present invention provides a system for improved bearing performance. The system includes a bearing assembly having a shaft member having a bearing race. A piezoelectric member opposing the bearing race is situated so that when a voltage is applied to it, it expands against the bearing race so that a preload of the bearing is varied. In one embodiment, the system includes a rotor having two outer bearing races opposing a shaft""s two inner bearing races. The contact angle for the first opposing bearing races is different than for the second opposing bearing races. In one embodiment, four piezoelectric elements are attached to the assembly so that they oppose the inner bearing race. In a further embodiment, a control system is coupled to the bearing assembly for providing controlled variation of the piezoelectric members.
In one embodiment of the present system, the piezoelectric member is disposed between the shaft member and the bearing race for varying the radial position of the bearing race. In another embodiment, the piezoelectric member is disposed for varying the axial position of the bearing race.
In one embodiment, the system includes a hydrodynamic bearing assembly having a stator member having a hydrodynamic surface and a rotor member having a surface facing the stator member hydrodynamic surface. The rotor member surface and the stator member hydrodynamic surface having a gap between them when the rotor surface is moving. A piezoelectric member is attached to either the stator member or the rotor member for varying the width of said gap between said surfaces. In a further embodiment, a control system is coupled to the bearing assembly for providing controlled variation of the piezoelectric members.
Therefore, the present system provides a user with a disc drive that has a bearing assembly in which the gyroscopic vibration can be actively damped, the preload can be varied as need, and the designer has active control of the power dissipation/stiffness tradeoff and the stiffness/life span trade-off. The disc drive is thus more dependable and has a longer life.